1. Field of the Invention
The present invention relates to an ink jet printing head adapted for use in an ink jet printer and a method for producing the same, and more particularly to an ink jet printing head in which, in forming nozzles and liquid chamber, the frame of the liquid chamber is constituted by the combination of walls having a width substantially equal to that of the nozzle wall, thereby improving adhesion of a heater board and a top plate and also improving the stability of manufacture, and a method for producing such ink jet printing head.
2. Related Background Art
For use in the ink jet printing head there have been proposed nozzles of various shapes, one of which will be explained with reference to FIG. 6.
Referring to FIG. 6, a top plate 101 is formed from a silicon wafer which is cut and polished in such a manner that the upper face is constituted by the (110) crystalline plane. There are also shown a penetrating hole 102 constituting a liquid chamber or an ink reservoir, and a groove 103 for an ink discharging nozzle.
A silicon chip 108 is provided with a plurality of heat generating members (heaters) 109 and will be hereinafter called a heater board. The top plate 101 and the heater board 108 are adhered in a direction shown in FIG. 6 to form oblong nozzles between nozzles 103 and the surface of the heater board 108. In such adhering operation, the positions of both components are precisely adjusted in such a manner that a heater 109 is contained in each nozzle. Ink is supplied from an unrepresented ink tank, then guided to an ink liquid chamber 102 and reaches the interior of the nozzles 103. The heater board 108 is controlled by an unrepresented control circuit and each heater 109 is energized according to the print data. The above-mentioned control circuit may be provided on the heater board or formed on another substrate, and will not be explained further as it is not related to the principle of the present invention.
The heater 109 energized according to the print data generates heat, thereby heating the ink in the corresponding nozzle. The heated ink boils above a certain critical temperature, thus generating a bubble. The generated bubble grows within a short time of several microseconds and gives an impact force to the ink, whereby a part of the ink is strongly pushed out and lands on a printing medium such as paper. A printed image is obtained by repeating this process.
In the following there will be explained the method of producing the top plate, with reference to FIGS. 7A to 7H. In these drawings, the views at the right-hand side are those of the top plate 101 seen from the lower side (nozzle side) while those at the left-hand side are cross-sectional views of the top plate cut along a plane in the discharging direction.
FIG. 7A illustrates a silicon wafer. FIG. 7B shows the formation of an oxide film. FIG. 7C shows patterning of SiO.sub.2. FIG. 7D shows formation of a SiN film. FIG. 7E shows the patterning of SiN. FIG. 7F shows anisotropic etching of Si, wherein a numeral 102 indicates a liquid chamber. FIG. 7G shows elimination of SiN. FIG. 7H shows anisotropic etching of Si, wherein a numeral 103 indicates a nozzle.
FIG. 7A shows a silicon (Si) wafer 105 used as the material for forming the nozzle members, having a crystalline orientation &lt;110&gt; on the surface and &lt;111&gt; in the longitudinal direction of the nozzle. Both sides of the silicon wafer 105 are subjected to the formation of a thin silicon dioxide film 106 of a thickness of about 1 .mu.m as shown in FIG. 7B, by thermal oxidation or CVD (chemical vapor deposition). The silicon dioxide layer 106 serves as a mask layer in anisotropic etching of silicon. Then, with the ordinary photolithographic process, the silicon dioxide layer 106 is patterned into the shape of nozzles and liquid chamber on one side (lower face in the illustration) and into the shape of the liquid chamber on the other side (FIG. 7C). Then, on the nozzle forming side, a silicon nitride layer 107 is formed for example by CVD (FIG. 7D) and is patterned into the shape of the liquid chamber (FIG. 7E).
The wafer is then subjected to anisotropic wet etching by immersion in etching liquid such as 22% solution of TMAH (tetramethylammonium hydride) whereby the etching proceeds in exposed areas of silicon on both sides of the wafer, namely according to the shape of the liquid chamber and the etched portions from both sides are eventually connected to form penetrating holes. Then the silicon nitride layer on the nozzle face is eliminated by etching (FIG. 7G) to expose the nozzle pattern formed in the silicon dioxide layer 106 in the step shown in FIG. 7C, and anisotropic etching is executed again with TMAH whereby a portion corresponding to the nozzle is etched. In this operation, the liquid chamber etched in the step shown in FIG. 7F is also further etched, but the shape of the liquid chamber is little affected because the etching time for the nozzle is shorter than that for the liquid chamber. Otherwise it is also possible to shorten the etching time for the liquid chamber in consideration of the etching time required for nozzle etching, thereby eventually obtaining the liquid chamber of the desired shape.
However, with the anisotropic etching of the present invention, there can be obtained a nozzle with a rectangular cross section because the &lt;111&gt; plane perpendicular to the wafer surface is present in the ink discharging direction, but, in the longitudinal direction of the nozzle, there is no crystalline plane capable of stopping the etching, so that the wall between the nozzles is overetched in the longitudinal direction to form an acute angle shape. Consequently, in such overetched portion, there inevitably remains the thin silicon dioxide film constituting the mask layer. Such silicon dioxide film alone is removed, without damaging silicon, by blowing pressurized air, eventually containing water, to the wafer. For removing the film of about 1 .mu.m by blowing water with pressurized air, there is only required a pressure of 1 to 20 kgf/cm.sup.2. Otherwise the entire silicon dioxide film may be removed by wet etching employing the mixture of ammonium fluoride and hydrofluoric acid.
The top plate 108 prepared by the above-described process is shown in FIG. 8. In the patterning process for forming the liquid chamber, the both surfaces of the top plate chip are formed in similar shapes, but the pattern at the ink supply side, at the upper surface in FIG. 6, may be made smaller at such a level that the penetrating hole is formed by anisotropic etching. In fact a pattern smaller than at the nozzle side is preferred in order to ensure the connection with the unrepresented ink supply member or the strength of the wafer in forming the top plate.
As explained in the foregoing, anisotropic etching of silicon can be utilized in forming the structure of the top plate, providing high mass producibility since the top plate can be prepared in the state of a wafer. Also the nozzle preparation by photolithographic technology allows to obtain nozzles of a high density with a high precision.
However, the conventional method of preparing the top plate has been associated with the following drawbacks because the nozzle walls and the liquid chamber frames are significantly different in width.
In forming the nozzles by adhering the top plate, principally comprising of silicon, with the heater board, the adhesion is most simply achieved by spraying an adhesive material. This is achieved by spraying, on the surface of the top plate, mist of a resinous adhesive material adjusted in viscosity with diluting liquid and mixed with compressed air. The adhesive comprises a material of high chemical resistance such as polyether amide resin (for example HIMAL supplied by Hitachi Chemical Co.), and is to form a protective film on the inner wall of the nozzle simultaneously with the coating of the adhesive material on the bottom faces of the nozzle walls of the top plate.
However, with the progress of the ink jet printer toward the higher image quality with a nozzle density of 360 dpi or higher, the width of the nozzle wall becomes as small as 40 .mu.m or even smaller, and, for a nozzle density of 600 dpi, the width of the nozzle wall becomes as small as 10 .mu.m, which is much smaller than the wall width of the liquid chamber frame. If the adhesive material is spray coated as explained above in such structure, the coated thickness of the adhesive material may fluctuate as shown in FIG. 9, depending on the width of the lateral walls constituting the nozzle and that of the lateral walls constituting the liquid chamber. More specifically the thickness d1 of the adhesive material on the nozzle walls becomes smaller than the thickness d2 of the adhesive on the liquid chamber walls because the latter is larger. For example, in case of coating the adhesive material with a thickness of 10 .mu.m on the liquid chamber frame of the larger width, the adhesive can only be coated with a thickness of 2 to 5 .mu.m on the nozzle wall of a width of 10 .mu.m though this value depends to a certain extent on the viscosity of the adhesive. Also if the thickness of the adhesive is reduced, matching the width of the nozzle wall, the thickness becomes relatively difficult to control and may show fluctuation.
Such fluctuation in the coating thickness of the adhesive results in a fluctuation in the overflowing amount at the adhesion of the substrate. FIG. 12A schematically shows the state of adhesion in case the wall width fluctuates. Since the frame wall of the liquid chamber 2 is wider than the nozzle wall 104, a larger amount of the adhesive 110 overflows into the nozzle 103 at the adhering operation. Such overflowing adhesive 110 deforms the shape inside the nozzle 103, thereby deteriorating the ink flow therein or the ink discharging direction therefrom and, if the adhesive sticks on the heat generating member, the heat generating state for ink discharge may be varied to disable the desired ink discharge.
On the other hand, such fluctuation in the thickness of the adhesive may result in insufficient adhesion on the lateral walls of the nozzle in adhering the top plate and the heater board. Such insufficient adhesion may lead to a crosstalk between the nozzles at the ink discharge or color mixing between the liquid chamber of different colors in case of a color printing head.
For adhering the substrate, the Japanese Patent Application Laid-open No. 60-206657 discloses a technology of forming the nozzle walls and the liquid chamber walls with photosensitive resin on the heater board and adhering the top plate thereon. In order to prevent formation of a closed space at the adhering face between the top plate and the wall, the wall is so formed as to form a space open to the exterior.
In such configuration, however, since the top plate is coated on its entire surface with the adhesive material and is then adhered onto the walls, the surface of the adhesive is exposed in a part of the nozzle. The surface of the adhesive material is difficult to control and may deform the cross-sectional shape of each nozzle, thus eventually affecting the ink flow therein.